1. Field of the Invention
The present invention relates generally to a telemetry unit used with a downhole drilling system. More specifically, this invention relates to a downhole telemetry unit that is capable of locating an underground signal source based upon the received waveform. Still more specifically, the present invention relates to a system and method that precisely locates an underground signal source and reconstructs the signal path of the acoustic wave from the source to a downhole telemetry device.
2. Description of the Related Art
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. By using this information, the driller is able to more precisely determine the orientation of the bottomhole assembly and the type of formation through which the bottomhole assembly formation is drilling. The collection of information relating to conditions downhole, commonly referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drill string must first be removed or tripped from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers must make some decisions (such as the direction to drill, etc.) without sufficient information, or else incur the cost of tripping the drill string to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement is questionable as drilling mud begins to invade the formation surrounding the borehole.
Because of these limitations associated with wireline logging, there has been an increasing emphasis on the collection of data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections "real-time", as necessary, to optimize performance. Moreover, the measurement of formation parameters during drilling increases the integrity of the measured data. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly while the bottomhole assembly is in the well.
The measurement of formation properties during drilling of the well by LWD systems increases the timeliness of measured data and, consequently, increases the efficiency of drilling operations. While LWD data is valuable in any well, those in the oil industry have realized the special importance of LWD data in wells drilled with a steerable bottomhole assembly, as described in assignee's U.S. Pat. No. RE 33,751. Extraneous noise downhole greatly complicates the implementation of acoustic logging tools in a LWD system. Thus, the noise generated by drilling, the flow of mud through the drill string, the grinding of the drilling components, and other mechanical and environment noises present downhole interfere with the reception and isolation of transmitted acoustic waves.
Logging sensors commonly used as part of an LWD system are resistivity, gamma ray, gamma density, and neutron porosity sensors. The assignee and other companies are currently experimenting with and implementing acoustic measurement devices to determine the properties of the formation surrounding LWD systems. Two types of suitable acoustic sensors are hydrophones and triaxial geophones. As is well known in the art, while a hydrophone may be used in the drill string, the type of information that can be detected with a hydrophone is limited to the measurement of pressure variations in fluids. In contrast, a geophone with three-dimensional capabilities provides more information, but must maintain contact with the wall of the well bore.
Modem petroleum drilling and production operations often require drilling from one well towards another well in which case the target well must be found and hit. Other applications require drilling one well while staying a specified distance away from another well in which case the second well must be found and tracked.
FIG. 1 shows a plan for joining two adjacent wells with well 110 being drilled while well 100 is the target. The inherent difficulties of joining wells 100 and 110 head-on can be appreciated. The target well 100 may only be 5 inches in diameter, the borehole from which well 110 is drilled may initially be over a mile away, and the intended intersection point may be five miles below the earth's surface.
The reasons for joining two wells vary. For example, two wells may be joined to increase production, thermal energy, or simply as a method of laying pipeline. Alternately, two wells may need joining to kill an old well. For example, as shown in FIG. 2, salt water may be leaking through an old casing contaminating a fresh water aquifer. The problem for a driller is finding the exact position of the target well so that advanced kill techniques may be employed to halt the contamination. To complicate matters, it is not always possible to place a source down the target well from the surface, because the top portion of the well may not be accessible.
It may also be important to keep a fixed distance from an adjacent target well. For example, FIG. 3 shows a well plan with a complicated herring-bone structure. As can be seen, maintaining a fixed distance from an adjacent well is required. FIG. 4 shows a highly complex well pattern in which it may be important to stay a specified distance away from certain wells while intersecting another well.
The industry has attempted to solve the problem of locating an existing well from a borehole being drilled by using electromagnetic waves. An electromagnetic source is placed in the well being drilled and the resistivity of the surrounding medium is detected. When the well being drilled is proximate to the old well, the conductive casing inserted in the old well indicates the presence of the old well. Ilowever, this technique has several drawbacks. First, it is limited to close range applications. In addition, this technique may have difficulty establishing exactly where on the target well the well being drilled is juxtaposed. Thus, instead of hitting the bottom of the target well, the sensed section of the target well may be several hundred feet from the target point. Finally, this prior art technique requires that a casing be present in the existing well. Ideally, the driller of the new well would like to know the exact relative location of a target in the existing well. Further, the further away that the target can be detected, the better. Preferably, no casing would be required in the existing well. By providing exact relative location information, an operator could drill with greater speed and certainty.
Therefore, a need exists for a long distance ranging device to find a target downhole. Preferably, this device could be implemented as part of an LWD system. Ideally, this device could also be used with a geo-steering system to automatically steer the bottomhole assembly to the existing well. Further, the ideal technique would not require a controlled source but could also determine the distance to and location of a noise or random source. It would not be dependent on a conductive member being present in a target well, but could find a signal source regardless of the presence of a casing. Preferably, the device would utilize a ranging technique that could detect multiple sources. It also could account for any underground refractions or reflections by the transmitted signal, thereby establishing the shortest drilling distance to the target.